Green’s Dictionary of Slang

number two n.

[euph.]

1. [late 19C+] (mainly juv.) defecation; thus as v. to defecate.

2. [1950s] (W.I.) a large round dumpling [it is indented around its circumference to facilitate splitting it in half, but note sense 1].

3. [1950s+] (orig. milit.) second-in-command, second in rank.